OSSEOUS TISSUE. 9 



the latter are larger, but do not weigh as much in proportion^ 

 because the shell, or outer layer, is more expanded and thinner, 

 affordifflg greater surface for muscular attachments. Whereas, 

 in the thorough-bred, a greater density of bone is necessary to 

 withstand the intense concussion of speedy action ; therefore the 

 bones are increased in thickness of shell, affording greater strength, 

 without apparent increase in size. 



OSSEOUS TISSUE. 



In bone tissue there are two modifications of texture, the com- 

 pact and the cancellated. The former — hard, dense, and ivory- 

 like — is always situated externally ; the latter — porous and 

 spongy — lies within. 



Although the compact -__^i1ii^>/i'-=^^^s:^- 



tissue appears uniformly 

 dense, and destitute of por- 

 osity, yet, if we transversely 

 sectate the shaft of a long 

 bone, and examine it under 

 the microscope, by trans- 

 mitted light, it is found to 

 contain numerous round 

 openings. These are called 

 Haversian canals. They 

 transmit blood-vessels, and 

 run in a longitudinal or 

 slightly oblique direction, 



opening on either the outer the peripheral and mterstitiallamellffi. 



or inner surface of the 



bone ; they also have many transverse branches of communica- 

 tion, which are often of greater diameter than the trunks. These 

 canals are from ^th to iTrrffth of^an inch in diameter, and 

 surrounded by concentric layers or lamellce of bone, which are 

 shady or opaque, and vary in number from two or three to six, the 

 internal being most distinct, each succeeding one becoming less so. 

 Between them may be seen many dark, irregulavly-oval objects 

 — the minute reservoirs or lacunce, containing the hone-corpus- 

 cles, from which numerous radiating processes occupy the- 

 canal icul i ; the latter are minute canals, which establish a. 

 communication between the Haversian canals and the lacun&.. 

 On the external part of the shell of the compact tissue of 

 long bones are several concentric osseous layers, the peripheral 



Fig. 2. 

 Transverse section of compact osseous tissue. An 

 Haversian canal, 3, is seen surronnded by concentric 

 layers, forming an Haversian cylinder, 1. The dark 

 objects between the lamella are lacunae ; 2 is a similar 

 cylinder, showing also the canalicuU ; 4 4, Portions of 



